MerTK is a member of a receptor tyrosine kinase (RTK) family known as TAM, which also includes AXL and TYRO3. Each member of the TAM family contains an extracellular domain, a transmembrane domain and a conserved intracellular kinase domain. MerTK was first discovered in the laboratory of H. Shelton Earp at the University of North Carolina in 1994 (Graham et al., Cloning and mRNA expression analysis of a novel human proto-oncogene, c-mer. Cell Growth Differ 5, 647-657 (1994)). The TAM family members undergo ligand-induced homodimerization, followed by catalytic tyrosine kinase activation and intracellular signaling. Cross-phosphorylation has also been demonstrated within this RTK family, suggesting heterodimerization can occur also. These RTKs are widely expressed in many epithelial tissues and in cells of the immune, nervous, and reproductive systems. MerTK was given its name by the Earp laboratory because it was found to be expressed in monocytes and in tissues of epithelial and reproductive tissue.
As described in more detail below, ligand-bound MerTK can complex with phosphatidyl serine and it binds apoptotic cells which triggers ingestion and suppresses inflammatory cytokines. It is aberrantly expressed in certain cancers (for example, acute leukemia (ALL and AML) and some solid tumors (for example melanoma, breast cancer, colon cancer, non-small cell lung carcinoma, glioblastoma and others).
The MerTK ligands include growth arrest-specific 6 protein (GAS6; Chen, et al; Oncogene (1997) 14, 2033-2039), protein-S, tubby and tubby-like protein-1 (TULP1), and galectin-3. Several of these ligands are present in serum and expressed locally in a number of tissues. These ligands bind to the extracellular domain of MerTK, resulting in tyrosine kinase activation.
Since the discovery of MerTK in the Earp laboratory in 1994, there has been a growing body of literature and patents that suggest the possibility of MerTK as a druggable target for a number of indications.
TAM receptor tyrosine kinases have been investigated for their involvement in certain infectious diseases. Shimojima, et al., reported the involvement of members of the Tyro3 receptor tyrosine kinase family, Axl, Dkt and MerTK, in the cell entry of filoviruses Ebolavirus and Marburgvirus, and concluded that each Tyro3 family member is likely a cell entry factor in the infection (“Tyro3 Family-mediated Cell Entry of Ebola and Marburg Viruses” Journal of Virology, October 2006 p. 10109-10116).
U.S. Pat. No. 8,415,361 to Lemke, et al. (claiming priority to a Nov. 9, 2007 provisional application), assigned to The Salk Institute for Biological Studies, describes the use of TAM receptor inhibitors as antimicrobials. In particular, the '361 patent reports that inhibition of the TAM pathway in virally infected macrophages from TAM triple knock-out mice leads to reduced levels of infection with a variety of pseudotyped viruses with either filoviral, retroviral or rhabdoviral glycoproteins. Brindley, et al., reported that in a bioinformatics-based screen for cellular genes that enhance Zaire ebolavirus (ZEBOV) transduction, AXL mRNA expression strongly correlated with ZEBOV infection (“Tyrosine kinase receptor Axl enhances entry of Zaire ebolavirus without direct interactions with the viral glycoprotein” Virology, 415 (2011) 83-84).
Morizono, et al, published that Gas6 mediates binding of the virus to target cells by bridging virion envelope phosphatidyl serine to Axl on the target cells. Replication of vaccinia virus, which was previously reported to use apoptotic mimicry to enter cells, is enhanced by Gas6, and Morizono asserts that these results reveal an alternative molecular mechanism of viral entry that can broaden host range and enhance infectivity of enveloped viruses (“The Soluble Serum Protein Gas6 Bridges Virion Envelope Phosphatidylserine to the TAM Receptor Tyrosine Kinase Axl to mediate Viral Entry” Cell Host & Microbe 9, 286-298, 2011). In 2014, Morizono and Chen reported that virus binding by viral envelope phosphatidyl serine is a viral entry mechanism generalized to a number of families of viruses (Morizono and Chen, “Role of Phosphatidyl Receptors in Enveloped Virus Infection”, J. Virology Vol 88(8), 4275-4290 (Jan. 29, 2014)).
WO2013/124324 filed by Amara et al. (priority date Feb. 21, 2012), and assigned to Institut National De La Sante et De La Recherche Medicale, reports that Dengue virus is mediated by the interaction between phosphatidylserine at the surface of the Dengue viral envelope and TAM receptors present at the surface of the host cell, and that such interaction can be blocked, thereby inhibiting entry of Dengue into host cells. They also report that the interaction between phosphatidyl serine and TAM receptors is used by other flaviviruses such as Yellow Fever, West Nile and perhaps Chikungunya. Amara focuses on antisense, siRNA and antibody approaches.
Similarly, Bhattacharayya et al., reports that several human viruses, for example Ebola, Dengue, and HIV, externalize PtdSer on their capsid during budding and use phosphatidylserine to bind to and activate TAM RTKs in the presence of TAM ligands, allowing entry of the virus into cells and furthermore, activation of MerTK in macrophages in response to viral particles expressing PtdSer stimulates an anti-inflammatory cytokine profile as if apoptotic material was being ingested, thereby inhibiting the anti-viral immune response. Bhattacharayya et al observe that TAM receptors are engaged by viruses to attenuate type 1 interferon signaling (“Enveloped viruses disable innate immune responses in dendritic cells by direct activation of TAM receptors”, Cell Host & Microbe 14, 136,-147 (2013)). See also Meertens, L. et al. The TIM and TAM families of phosphatidylserine receptors mediate dengue virus entry. Cell Host Microbe 12, 544-557, doi: 10.1016/j.chom.2012.08.009 (2012). Mercer, J. & Helenius, A. Vaccinia virus uses macropinocytosis and apoptotic mimicry to enter host cells. Science 320, 531-535, doi:10.1126/science.1155164 (2008).
MerTK is ectopically expressed or overexpressed in a number of hematologic and epithelial malignant cells. Expression of MerTK and GAS6 correlates with poor prognosis and/or chemoresistance in these tumor types. The mechanisms by which increased MerTK signaling in tumor cells contributes to tumor malignancy, however, remain unclear.
WO2013/052417 titled “Pyrrolopyrimidine Compounds for the Treatment of Cancer” filed by Wang, et al., and assigned to the University of North Carolina describe pyrrolopyrimidines with MerTK inhibitory activity for the treatment of tumors such as myeloid leukemia, lymphoblastic leukemia, melanoma, breast, lung, colon, liver, gastric, kidney, ovarian, uterine and brain cancer, wherein the pyrrolopyrimidines have the general structures below, with R substituents as defined in the those applications:

In November 2013, Dr. Stephen Frye presented data showing the inhibitory effects of a pyrrolopyrimidine compound (UNC2025) in non-small cell lung cancer cell lines, MerTK-expressing AML cell lines, and MerTK-negative AML cell lines. In addition, the effect of UNC2025 was analyzed in an ALL 697 cell line xenograft model and a FLT3-ITD AML patient xenograft model. Frye, S. “Academic Drug Discovery and Chemical Biology”, Presentation at the Northwestern 18th Annual Drug Discovery Symposium. November 2013. The structure of the pyrrolopyrimidine compound UNC2025 is:

WO2011/146313 and WO2014/062774, both titled “Pyrazolopyrimidine Compounds for the Treatment of Cancer” filed by Wang, et al., and assigned to the University of North Carolina describe pyrazolopyrimidines with MerTK inhibitory activity for the treatment of tumors such as myeloid leukemia, lymphoblastic leukemia, melanoma, breast, lung, colon, liver, gastric, kidney, ovarian, uterine and brain cancer, wherein the pyrazolopyrimidines have the general structures below, with R substituents as defined in the those applications:
WO2014/062774 further discloses pyrazolopyrimidine compounds for use in a method of treating or inhibiting blood clot formation.
In January 2012, Liu, J, et al., published a comparison of the activity of forty four pyrazolopyrimidine compounds against MerTK, Axl and Tyro3 kinases. One of these compounds (UNC569) was tested for inhibition of MerTK autophosphorylation in human B-ALL cells (“Discovery of Novel Small Molecule Mer Kinase Inhibitors for the Treatment of Pediatric Acute Lymphoblastic Leukemia.” ACS Med Chem Lett. 2012 Feb. 9; 3(2):129-134.). In May 2013, Schlegel, et al., published results on the pyrazolopyrimidine compound UNC1062, which reduced activation of MERTK-mediated downstream signaling, induced apoptosis in culture, reduced colony formation in soft agar, and inhibited invasion of melanoma cells (“MER receptor tyrosine kinase is a therapeutic target in melanoma.” J Clin Invest. 2013 May; 123(5):2257-67).
In December 2013, Zhang, W., et al., also published a comparison of the activity of forty six 5-arylpyrimidine based compounds for treatment of tumors (“Pseudo-cyclization through intramolecular hydrogen bond enables discovery of pyridine substituted pyrimidines as new Mer kinase inhibitors.” J. Med. Chem., vol. 56:9683-9692, 2013). These pyrimidine compounds were identified using a pseudo-ring replacement strategy based on the previously identified pyrazolopyrimidine MerTK inhibitor, UNC569.
In July 2013, Liu, J, et al. published the first evidence of anti-tumor activity mediated by a member of this novel class of inhibitors. Specifically, the pyrazolopyrimidine compound UNC1062 inhibited MerTK phosphorylation and colony formation in soft agar (“UNC1062, a new and potent Mer inhibitor.” Eur J Med Chem. 2013 July; 65:83-93). In November 2013, Christoph, S. et al., published effects of a pyrazolopyrimidine (UNC569) in ALL and ATRT (atypical teratoid/rhabdoid tumors (ATRT) (“UNC569, a novel small-molecule Mer inhibitor with efficacy against acute lymphoblastic leukemia in vitro and in vivo.” Mol Cancer Ther. 2013 November; 12(11):2367-77). The MerTK inhibitors UNC569 and UNC1062 have the following structures:

An important observation was made in 2013 that MerTK −/− knock-out mice are less susceptible to tumor growth than normal mice. MerTK is normally expressed in myeloid lineage cells where it acts to suppress pro-inflammatory cytokines following ingestion of apoptotic material. It was found that MerTK −/− leukocytes exhibit lower tumor cell-induced expression of wound healing cytokines (IL-10 and GAS6) and enhanced expression of acute inflammatory cytokines (IL-12 and IL-6). Further, intratumoral CD8+ lymphocytes are increased. The loss of MerTK in the tumor microenvironment in Mer−/− mice slowed the establishment, growth, and metastasis of mammary tumors and melanomas in immune competent, syngeneic mice. Cook, R. S. et al., MerTK inhibition in tumor leukocytes decreases tumor growth and metastasis, J Clin Invest 123, 3231-3242 (2013).
Linger et al. have also presented data demonstrating increased MerTK expression in E2A-PBX11 and other cytogenetic subgroups of B-acute lymphoblastic leukemia (B-ALL), and that MerTK inhibition may attenuate prosurvival and proliferation signaling. Linger et al., Mer receptor tyrosine kinase is a therapeutic target in pre-B-cell acute lymphoblastic leukemia, Blood, vol. 122(9):1599-1609, 2013. Lee-Sherick, et al. (“Efficacy of a Mer and Flt3 tyrosine kinase small molecule inhibitor, UNC1666, in acute myeloid leukemia”, Oncotarget, Advance Publications 2015 Feb. 10, 2015) have reported that UNC 1666 (a pyrrolopyrimidine) decreases oncogenic signaling and myeloid survival in AML.
TAM (Tyro3-Axl-Mer) receptor tyrosine kinases have also been investigated for their involvement in platelet aggregation. In 2004, Chen et al, from the Johnson & Johnson Pharmaceutical Research and Development, published that MerTK, presumably through activation by its ligand Gas6, participates in the regulation of platelet function in vitro and platelet-dependent thrombosis in vivo. Chen, et al, “Mer Receptor tyrosine Kinase Signaling Participates in Platelet Function”, Arterioscler. Thromv. Vase. Biol. 1118-1123 June 2004. Chen reported that PtdSer on aggregating platelets activates MerTK, helping to stabilize clot formation. MerTK knockout mice have decreased platelet aggregation while maintaining normal bleeding times and coagulation parameters. Consequently, these mice appear to be protected from thrombosis without concomitant increased spontaneous bleeding (see also Angelillo-Scherrer A et al., Role of Gas6 receptors in platelet signaling during thrombus stabilization and implications for antithrombotic therapy. J. Clin. Invest. 2005, 115 (2), 237-246).
In 2007, Sather, et al., reported that membrane-bound MerTK is cleaved in the extracellular domain via a metalloproteinase to produce a soluble MerTK that decreased platelet aggregation in vitro and prevented fatal collagen/epinephrine-induced thromboembolism. “A soluble form of the Mer receptor tyrosine kinase inhibits macrophage clearance of apoptotic cells and platelet aggregation”, Blood, Vol 109(3): 1026-1033).
Paolino et al. have reported on the treatment of wild-type NK cells with a newly developed small molecule TAM kinase inhibitor, LDC1267, that conferred therapeutic potential and efficiently enhancing anti-metastatic NK cell activity in vivo. Oral or intraperitoneal administration using this TAM inhibitor markedly reduced murine mammary cancer and melanoma metastases dependent on NK cells. See, Paolino, M., et al., The E3 ligase Cbl-b and TAM receptors regulate cancer metastasis via natural killer cells, Nature, vol. 507:508-512, 2014. LDC1267 is a highly selective TAM kinase inhibitor with IC50 of <5 nM, 8 nM, and 29 nM for MerTK, Tyro3, and Axl, respectively, and has the chemical structure:

Bernsmeier, et al., have noted that characteristics of decompensated cirrhosis and acute-on-chronic liver failure (ACLF) include susceptibility to infection, immune paresis and monocyte dysfunction. The authors found that the number of monocytes and macrophages that expressed MerTK was greatly increased in circulation, livers and lymph nodes of patients with ACLF. They found that addition of a substituted pyrazolopyrimidine UNC569 (see WO 2011/146313 filed by Wang, et al., and assigned to University of North Carolina at Chapel Hill, page 25) restored production of inflammatory cytokines. Bernsmeier, et al., “Patients with Acute-on-Chronic Liver Failure Have Increased Numbers of Regulatory Immune Cells Expressing the Receptor Tyrosine Kinase MERTK”, Gastroenterology 2015; 1-13.
It is an object of the invention to identify new methods and compositions for the treatment of infectious diseases.
It is another object of the invention to identify new methods and compositions for the treatment of thrombosis.
It is another object of the invention to identify new methods and compositions for the treatment of a tumor, cancer or other neoplasm.
It is yet another object of the invention to identify new methods and compositions for the treatment of disorders that can be treated with immunosuppression, or which would benefit from immunostimulatory therapy.